The present invention relates to a dielectric gas mixture for use in electrically insulating a conductor.
When high voltages exist between the conductors of electrical apparatus (e.g., transformers, circuit breakers, or switches), arcing or sparking may take place. To prevent this phenomenon, dielectric fluids (gas or liquid) or solids are conventionally used to insulate the conductors.
One well-known dielectric gas is sulfur hexafluoride (SF.sub.6). While possessing good arc interrupting properties it is relatively expensive and suffers from relatively low vapor pressure at low temperatures and a comparatively high freezing point. Many dielectric gas mixtures have been suggested, such as illustrated in Mears et al. U.S. Pat. No. 4,071,461.
Efforts have been made to improve the cost-effectiveness of circulating SF.sub.6 gas in power transmission lines and substations by diluting the SF.sub.6 with nitrogen. Such dilution has been found to result in a substantial cost reduction but with a small reduction in the uniform-field electric strength. This reduction in electric strength is consistent with a linear addition of the net ionization and attachment coefficients of the two gases. This rule or formula was proposed first by A. Wieland, ETZ-A, 94 (1973), pages 370-373. A similar, but less marked effect, occurs with SF.sub.6 and He. This assumption that ionization and attachment coefficients add linearly in gas mixtures had been assumed to be accurate by authors in the field.